gluon-core: add functions to unset sysconfig settings

package/gluon-core/files/lib/gluon/functions/sysconfig.sh

@@ -12,3 +12,7 @@ sysconfig_isset() {
 sysconfig_set() {
 	echo -n "$2" > "$SYSCONFIGDIR/$1"
 }
+
+sysconfig_unset() {
+	rm -f "$SYSCONFIGDIR/$1"
+}

package/gluon-core/files/usr/lib/lua/gluon/sysconfig.lua

@@ -11,10 +11,13 @@ local function get(_, name)
 end
 
 local function set(_, name, val)
-	local ret = nil
+	if val then
 		local f = io.open(sysconfigdir .. name, 'w+')
 		f:write(val)
 		f:close()
+	else
+		os.remove(sysconfigdir .. name)
+	end
 end
 
 local setmetatable = setmetatable